Antistatic synthetic resin articles are used in their applications which dislike adhesion and deposition of dusts and other fine particles and hence provided with antistatic properties on the article surfaces in order to scatter electrostatic charge on the surfaces.
In order to impart antistatic properties to the synthetic resin molded articles, conventionally, conductive microparticles or fibers of stannic oxide, carbon, etc. are formulated and uniformly dispersed in the resin. The antistatic properties are produced by mutual contacts among the microparticles and/or fibers, thereby reducing the electric resistance of the molded articles.
Also, the applying of the antistatic property to only the surface of the molded article has been performed by including the conductive particles or fibers in only a surface layer of molded articles. For example, the molded articles have been produced by applying, to the surface of a resin substrate, a coating layer or film formed which includes the conductive particles or fibers and some synthetic resin, these molded articles being utilized as antistatic resin plates or the like.
In the conventional antistatic resin plates, a thermoplastic resin substrate is molded and the above antistatic film is then laminated on the substrate, and thereby the plates are obtained on which the surface exhibits high antistatic properties with the surface electric resistivity of 10.sup.11 .OMEGA./.quadrature. or less.
However, as the resin plate formed in this manner is further heated and deformed in a manner that the surface area is extended, for example, by bending or vacuum forming, the surface resistance after the secondary forming is increased, posing the problem of a reduction in antistatic properties. The surface electric resistivity is generally increased by the extending deformation of antistatic resin plate. It is considered that this is caused by reduced frequencies of mutual contact among the microparticles or fibers or increased intervals between the particles due to extension of the resin in which the conductive particles or fibers are dispersed.
When the deformation in the forming step is so large as to produce plastic deformation, it is observed that the surface resistivity is increased since the conductive fibrous materials are oriented in the deforming direction to decrease the frequency of the mutual contact of the fibers and also the frequency of intervals of the fibers needed for the conductivity. This tendency will be clearly significant if the conductive materials are stiff and short fibers.
The molded article, which increases in surface resistivity with the antistatic properties decreased when the antistatic resin plate is formed such as a secondary processing after molded, must have limitations to its uses. In this case, the antistatic properties can be imparted by applying an antistatic paint to desired positions on the surface of the molded article after forming the molded article from the normal thermoplastic resin plate, whereas this requires any steps of applying the antistatic paint in which in the molded particles with complicated shapes, the formation of uniform coating layers having excellent surface qualities is difficult, no uniform antistatic properties being imparted.
In view of the above problems, the present invention has an object of providing an antistatic resin molded article having thermal-formability, which is not accompanied by a substantial increase in surface resistivity even by forming after imparting antistatic properties therein.